1. Field of the Invention
The invention relates to methods for detecting hydrocarbons in geological strata and, more particularly, to detection methods which use the analysis of drilling muds.
2. Description of the Related Arts
A number of methods are currently used for detecting the composition of geological strata. In this manner, oil exploration uses analyses of rocks extracted by means of exploratory drilling operations, seismological analyses, et cetera.
For a number of years, studies have been carried out in order to demonstrate the advantage of analyzing the contents of hydrocarbon gases in drilling muds.
The oldest method was described by Pixler, B. O. in 1969 in “Formation Evaluation by Analysis of Hydrocarbon Ratios”, (Journ. Pet. Tech., vol. 21, no 6) then supplemented by Ferrie in 1981 (Ferrie, G. H., Pixler, B. O., and Allen, S., “Wellsite Formation Evaluation by Analysis of Hydrocarbon Ratios”, 83rd Ann. Meeting Ca. Soc. Min. & Met., 1981). It involves establishing the ratios between methane (C1) and different gaseous hydrocarbons, such as ethane (C2), propane (C3) and butanes (C4+). The potential features of the analyzed zone are established from these ratios. The applicant thus considers, in the earth analyses carried out up to the present time, that a C1/C2 ratio of between 15 and 65 indicates a zone which is rich in gas.
However, the method described by Pixler, and the variants thereof, do not allow a meaningful representation of a large volume of data.
Therefore, whilst still using the contents of light alkane in the drilling muds, other more complex ratios have been sought because they are more representative of the hydrocarbon contents of geological strata (Whittaker, M., Sellens, M., Analysis uses alkane ratios from chromatography, Oil & Gas Journal, May 18, 1987).
U.S. Pat. No. 2,938,117 belonging to K. H. Schmidt thus describes a method for analyzing the hydrocarbon contents of various wells by means of analysis of the composition of the gases of the wells.
This patent indicates that, by establishing a log-log graph of the ratio representing the ratio of two types of gaseous hydrocarbons in accordance with the gas/liquid ratio of the corresponding well, it is possible to represent the developments of this ratio between gases on a straight line. Provided with this chart, the detection of a value of a ratio between two gases extracted from a well thus allows the gas/liquid ratio of the well to be determined.
However, the unreliability of the measurements linked to the analysis system and the differences in the quality of the drilling mud leads to a level of unreliability or bias which makes the interpretation of these ratios difficult and subject to a number of errors.